This application is based on Japanese Patent Application No. 2001-008934 filed on Jan. 17, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a hydraulic pressure control apparatus for an automatic transmission of an automotive vehicle, and more particularly to techniques for controlling an electrically operated hydraulic pump provided as a hydraulic pressure source.
2. Discussion of Related Art
An automatic transmission for an automotive vehicle is provided with hydraulic actuators operable to change its speed ratio, and a hydraulic pressure control apparatus for controlling the hydraulic actuators. This hydraulic pressure control apparatus includes an electrically operated hydraulic pump which functions as a hydraulic pressure source and which is operated by an electric motor. The hydraulic pressure control apparatus is arranged to control the operating speed of the electrically operated hydraulic pump while taking account of the required delivery pressure, the required rate of delivery (including a leakage flow), and the operating efficiency of the pump, so as to minimize the operating speed of the electric motor, for thereby minimizing the required amount of consumption of electric power by the electric motor and the operating noise of the electric motor. The hydraulic pressure control apparatus permits the hydraulic pump to generate a sufficiently high hydraulic pressure available for use on an electric or hybrid vehicle which uses an electric motor as a drive power source.
The above-described hydraulic pressure control apparatus for the vehicle automatic transmission is arranged to control the operating speed of the electric motor on an assumption that the hydraulic pump is continuously operated. This arrangement does not necessary permits an adequate control of the hydraulic pump when the hydraulic pump is initially turned on while it is at rest. Namely, upon starting of the hydraulic pump, hydraulic pipes or conduits connected to the hydraulic pump must be filled with a working fluid, with complete removal of air cavities or bubbles from a mass of the working fluid in the pipes, so that the hydraulic pump becomes actually capable of delivering a pressurized fluid according to its nominal capacity. Accordingly, the capacity of the hydraulic pump is not available during a predetermined initial period of operation of the hydraulic pump. This drawback is serious particularly at a relatively low operating temperature of the hydraulic pump at which the working fluid has a relatively high degree of viscosity.
In view of the drawback described above, there has been proposed a hydraulic pressure control apparatus arranged to temporarily increase the operating speed of the hydraulic pump during an initial period of operation immediately after the starting, so that the hydraulic pump can be operated with its nominal capacity immediately after its starting. JP-A-2000-27763 discloses an example of such a hydraulic pressure control apparatus as applied to an automatic transmission of an automotive vehicle. This hydraulic pressure control apparatus is arranged such that the length of the initial period of operation during which the operating speed of the hydraulic pump is temporarily increased is determined depending upon the temperature of the working fluid, so that the hydraulic pump can be rapidly primed to fill the hydraulic conduits upon starting of the hydraulic pump while it is at rest. This arrangement is effective to minimize the required time of delay from the moment of starting of the hydraulic pump to the moment at which the hydraulic pump becomes capable of producing the nominal output.
The above-described conventional hydraulic pressure control apparatus for the automatic transmission of the automotive vehicle is simply adapted to change the initial high-speed operating period depending upon the operating temperature of the working fluid, for reducing the required delay time from the moment of starting of the hydraulic pump to the moment at which the hydraulic pump becomes operable with the nominal capacity. However, the conventional apparatus is not arranged to control the rate of increase of the operating speed of the hydraulic pump during the initial high-speed operating period. That is, the resistance to rotation of the hydraulic pump increases with an increase in the viscosity of the working fluid, which increases with a decrease of the temperature of the fluid. If the operating speed of the hydraulic pump were increased during the initial period at the same rate as in the subsequent period of normal operation of the pump, vanes of a rotor of the pump, for example, might be damaged due to a stress caused by the resistance to the rotation of the rotor. Alternatively, a suction passage connected to the hydraulic pump might suffer from cavitation due to a rapid increase of the fluid flow velocity caused by a rapid increase of the operating speed of the pump, resulting in a problem of operating noise or air suction into the pump.
It is therefore an object of the present invention to provide a hydraulic pressure control apparatus for an automatic transmission of an automotive vehicle, which apparatus is arranged so as to prevent a damage of the vanes of the rotor or other components of the hydraulic pump which may otherwise arise upon starting of the pump when the working fluid has a relatively low temperature.
The above object may be achieved according to the principle of this invention, which provides a hydraulic pressure control apparatus for controlling an automatic transmission of an automotive vehicle, with a pressurized working fluid delivered from a hydraulic pump driven by an electric motor, the apparatus including temperature detecting device for detecting a temperature of the working fluid, and pump control means operable upon starting of the electric motor to start the hydraulic pump, for controlling the electric motor, such that a rate of increase of the operating speed of the hydraulic pump decreases with a decrease in the temperature of the working fluid detected by the temperature detecting device.
In the hydraulic pressure control apparatus constructed according to the present invention as described above, the electric motor is controlled by the pump control means, to drive the hydraulic pump such that the rate of increase or rise of the operating speed of the hydraulic pump immediately after its starting decreases with a decrease in the temperature of the working fluid detected by the temperature detecting device. Accordingly, the present hydraulic pressure control apparatus is effective to prevent the problems conventionally encountered upon starting of a hydraulic pump to start an automotive vehicle at a relatively low ambient temperature at which the working fluid has an accordingly high degree of viscosity. That is, the present hydraulic pressure control apparatus effectively prevents damaging of the vanes of a rotor of the hydraulic pump due to an excessively high load acting thereon, and cavitation noises and air suction on the suction side of the hydraulic pump.
According to one preferred form of the present invention, the pump control means is arranged to control the electric motor such that the operating speed of the electric motor is held at a predetermined first value for a predetermined first time after the starting of the hydraulic pump, and is then held at a predetermined second value lower than the predetermined first value, for a predetermined second time. The predetermined first value is preferably substantially equal to a predetermined upper limit of the operating speed at which the hydraulic pump provides its maximum delivery, or a value which is lower than the upper limit by a predetermined amount. According to the present arrangement of the pump control means, the operation of the hydraulic pump at the predetermined first speed value for the predetermined first time permits rapid filling of the hydraulic pipes in the hydraulic pressure control apparatus with the working fluid, making it possible to effectively reduce a required length of time from the moment of starting of the electric motor to the moment at which the delivery of the pressurized fluid from the hydraulic pump is started.
According to one advantageous arrangement of the above-indicated preferred form of the present invention, the hydraulic pump is a vane pump having an inner circumferential surface partially defining a pump chamber, the vane pump including a rotor having an outer circumferential surface, eccentrically disposed in the pump chamber and rotated by the electric motor, and a plurality of vanes which are supported by the rotor such that the vanes are arranged and movable in a radial direction of the rotor, for sliding contact, at outer ends thereof projecting from the outer circumferential surface, with the inner circumferential surface. In this advantageous arrangement, the above-indicated predetermined second value of the operating speed of the hydraulic pumps and the predetermined second time for which the operating speed is held at the predetermined second value may be preferably determined so as to enable the vanes to be moved for sliding contact of the outer ends with the inner circumferential surface. In the present form of the invention, the vanes can be rapidly moved into sliding contact with the inner circumferential surface partially defining the pump chamber, upon starting of the electric motor, even when the temperature of the working fluid is relatively low.
According to another preferred form of this invention, the automatic transmission includes a hydraulically operated frictional coupling device, and the pump control means changes the operating speed of the hydraulic pump from the predetermined first value to the predetermined second value upon determination that the frictional coupling device has been brought to an engaged state as a result of starting of said hydraulic pump, more precisely, as a result of completion of filling of the hydraulic pipes in the hydraulic control apparatus with the working fluid after the starting of the hydraulic pump. This arrangement assures a required minimum time during which the operating speed is held at the predetermined first value, making it possible to avoid an unnecessarily high load on the electric motor.